In recent years, electric power steering apparatuses have increasingly been used to provide a comfortable steering touch or feel to the driver by reducing the manual steering effort or force needed to turn the steering wheel. The electric power steering apparatuses of the type concerned are constructed such that an assist torque generated by an electric motor in proportion to the steering torque is transmitted to the rack shaft of a steering system. A typical example of such known steering apparatuses will be described below with reference to FIGS. 11A and 11B.
As shown in FIGS. 11A and 11B, the conventional electric power steering apparatus 200 includes a rack shaft 201 extending in a transverse direction of the vehicle and having a rack 202 of gear teeth formed at its one end portion, a pinion shaft 203 having a pinion 204 meshing with the rack 202, a ball-screw mechanism (often referred to as “ball-nut mechanism”) 206 having a nut 207 engaged theadedly, via balls 208 (FIG. 11B), with a worm screw 205 formed at the other end portion of the rack shaft 201, and an electric motor 211 having a motor shaft 213 (FIG. 11B) connected to the nut 207.
The pinion shaft 203 forms a steering shaft connected to a steering wheel (not shown), and the worm screw 205 is a mail or externally threaded screw extending over a predetermined length of the rack shaft 201 from the other end (left end in FIG. 11A) thereof. The rack shaft 201 is slidably received in an elongate housing 221 which is composed of a first housing 222 on the rack side (right-hand side in FIG. 11A) and a second housing on the electric motor side (left-hand side in FIG. 11A). On the rack shaft 201, the electric motor 211 is located between the rack 201 and the nut 207. With this arrangement, an assist torque generated by the electric motor 211 in proportion to the steering torque applied to the steering wheel is transmitted via the nut 207 to the rack shaft 201.
As shown in FIG. 11B, the first housing 222 supports one end portion 213a (right end in FIG. 11B) of the motor shaft 213 of a rotor 212 via a bearing 224 such that the motor shaft 213 is not slidable but rotatable relative to the first housing 222. The second housing 223 rotatably supports the other end portion 213b (left end in FIG. 11B) of the motor shaft 213 via a bearing 225. The motor shaft 213 is a hollow shaft surrounding a longitudinal portion of the rack shaft 201, and the nut 207 is assembled integrally with the end portion 213b of the motor shaft 213. In FIG. 11B, reference numeral 231 denotes a stator of the electric motor 211. Reference numerals 232 and 233 denote a lock nut for the bearing 224 and a lock nut for the motor shaft 213, respectively. Reference numeral 234 denotes a stopper for positioning the motor shaft 213. Reference numeral 235 denotes a lock nut for the nut 207. Reference numerals 236 and 237 denote a cylindrical commutator of the rotor 212 and a brush of the stator 231, respectively. Reference numeral 238 denotes a bolt firmly connecting together the first and second housings 222, 223.
As discussed above with reference to FIG. 11B, the conventional electric power steering apparatus 200 (FIG. 11A) is arranged such that the electric motor 211 is disposed between the rack 201 on the first housing 222 side and the nut 207 on the second housing 223 side, with one end portion 213a of the motor shaft 213 being non-slidably supported by the first housing 222, the other end portion 213b of the motor shaft 213 being supported by the second housing 223, and the nut 207 being supported by the other end portion 213b of the motor shaft 213
The conventional electric power steering apparatus 200 of the foregoing construction is generally assembled and inspected in a manner, as described below.    (1) Structural parts or members of the electric motor 211 are assembled in the second housing 223. In this instance,the second housing serves also as a motor case.    (2) The electric motor 211 is subjected to a quality inspection and performance test, and after that the electric motor 211 is disassembled.    (3) The nut 207 is firmly attached to the end 213b of the motor shaft 213.    (4) The end 213a of the motor shaft 213 is assembled in the first housing 222 via the bearing 224 and the related parts 232-234.    (5) The structural members of the electric motor 211 and the second housing 223 are assembled with the first housing 222.    (6) The rack shaft 201 is assembled in the housing 221 so that an electric power steering apparatus 200 is produced.    (7) The electric motor 211 is solely subjected again to a quality inspection and performance test.    (8) The electric power steering apparatus 200 as a whole is subjected to a quality inspection and performance test.
As thus for explained, the conventional electric power steering apparatus 200 requires assembly of the electric motor 211 for the quality inspection and performance test, followed by disassembly of the electric motor 211 for the assembly of the rack shaft 201 and ball-screw mechanism 206 with the electric motor 211. This increases the assembling and inspection man-hours of the electric power steering apparatus 200, which leads to an increase in cost. Thus, there is room for improvement in the conventional electric power steering apparatus 200.